Renewable materials will play an increasingly important role in the future. Biological conversion processes will be important in producing valuable products, such as ethanol, from renewable starting materials, such as biomass. A bottle-neck in converting biomass is formed by the step in which the release of carbohydrates from lignocellulose, which is the main constituent of biomass, is to be carried out. In order to allow biocatalysts (e.g. microorganisms) access to the starting material, it is necessary to depolymerize and/or decrystallize the lignocellulose.
A known technique for this purpose is strong acid treatment, which is described for instance in U.S. Pat. Nos. 5,562,777, 5,580,389, 5,597,714, 5,620,877, 5,726,046, 5,782,982, 5,820,687, 6,054,611 and 6,239,198.
WO-A-94/23071 describes a method for producing sugars from materials containing cellulose and hemicellulose. The method comprises decrystallization and hydrolysis of the cellulose and hemicellulose with an acid and separation of the hydrolysate into sugars and acid.
U.S. Pat. No. 4,427,584 describes a stepwise process for converting crystalline α-cellulose to amorphous α-cellulose with liquid or gaseous sulphur trioxide to cause decrystallization of the α-cellulose followed by hydrolysis of the treated fibers to produce sugars.
The known techniques for converting cellulosic materials into fermentable products usually require the presence of a mechanical stirring device to obtain sufficient mixing of the cellulosic material and the strong acid. In practice this often leads to complications, because of the combination of moving parts and strong acid, which gives rise to corrosion problems. The cellulosic materials also generally require mechanical pretreatment to obtain the required particle size (usually 10 mm in diameter, or less), and water content (in general less than 10%).
Furthermore, the known techniques require chopping or grinding of the cellulosic materials to a particle size not greater than 10 mesh, in order to overcome the difficulty of providing good contact between the strong acid and the cellulosic material. This small particle size is also required to allow for suspension of the cellulosic materials in a hot air stream.
Furthermore, a separate evaporator is necessary to produce concentrated acid. This results in extra investment costs.
Furthermore, the known techniques referred to require a considerable amount of acid, which is subsequently neutralized using an appropriate amount of base. This leads to considerable streams of waste, such as gypsum, that need to be disposed of and thus incur costs.